it depends. Are you supplementing them with foods like algae wafers? Do you have algae in your tank?
If you are getting ottos to eat algae, and you don't have any algae, then don't get the ottos haha. It seems to be a misconception that I had fallen into myself. There is nothing to say you need ottos in a tank. If there is algae they are helpful for keeping it under control but fixing the root problem is far more effective.
To directly answer your question, yes there is enough room given your stocking for one or two more ottos. My 20g long has two in there and they seem well fed- I drop in algae wafers and they usually don't even touch it.